Microwave modulator



July 21, 1959` J. c. cAcHx-:Rls ET AL 2,896,172

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Tousj .Jo/m 6. Cache/s Herbert A.

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United States Patent O MICROWAVE MonULATon John C. Cacheris, Bethesda, Md., and Herbert A. Dropkin, Washington, D.C., assignors to the United States of America as represented by the Secretary of the Army Application March 6, 1957, Serial No. 644,425

2 Claims. (Cl. 332-45) (Granted under Title 35, U.S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to us of any royalty thereon.

This invention relates to apparatus for use at microwave frequencies and in particular to single sideband, suppressed carrier modulators.

In this invention a single stream of energy in the microwave frequency range is divided into a plurality of equal energy streams, the streams are shifted in phase in relation to each other, the streams are then amplitude modulated by longitudinally magnetized ferrite means, and certain sidebands of the modulated streams are added together to give as an output a single sideband of a frequency of the parent stream plus (or minus) the modulation frequency, and with suppression of the carrier.

It is an object of this invention to provide means whereby a parent stream of energy at microwave frequencies is modified, and in the modified state is modulated by longitudinally magnetized ferrite means, and selected sidebands are next added together to give as an output a single sideband of a frequency other than that of the parent stream and with the carrier suppressed so that it does not appear in the output.

It is an object of this invention to provide means whereby a stream of energy at microwave frequencies is divided into a plurality of streams of energy and of differing phase relations which are each amplitude modulated by ferrite modulators in the range of audio frequencies, and selected sidebands of the modulated streams are added together to give as an output a single sideband having the frequency of the parent stream together with the modulation frequency, and with suppression of the carrier so that it does not appear in the output.

It is an object of-this invention to provide means whereby `a parent stream of energy at microwave frequencies is divided into a plurality of streams of energy of differing phase relation, to amplitude modulate these streamsl by ferrite modulators, and to reflect back into the path of the parent stream selected sidebands in added relation to give as an output a single sideband having a frequency of the parent stream together with the modulation frequency, and with the carrier suppressed so that it does not appear in the output.

It is an object of this invention to provide amplitude modulation of a stream of energy at microwave frequencies by means of a longitudinally magnetized ferrite modulator, to change the frequency of the modulated stream so that the output from the modulator is of a frequency different from the input to the modulator.

The specific nature of the invention as well as other objects, uses and advantages thereof will clearly appear from the following description and from the accompanying drawing, in which: Y

Figure 1 is a diagrammatic showing of a transmission system for the production of a single sideband output from an entering microwave carrier of a given frequency,

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and with suppression of the carriers, and with the single sideband output being of afrequency different from that of the carrier.

Figure 2 is a view in plan of a ferrite modulator and connected guide sections used in the showing in Figure l.

Figure 3 is a plan view of the ferrite modulator.

Figure 4 is a wiring diagram of the means for magnetizing the cores of the ferrite modulators.

Figure 5 is a vector diagram illustrating the relationship between the modulated carriers and their sidebands.

Figure 6 is a diagrammatic showing of a reflection system for the production of a single sideband output reflected back through the input of the system.

Figure 7 is a diagrammatic showing of a two-phase transmission system for the production of a single sideband output at a changed frequency and with suppression of the carriers.

Figure 8 is a vector diagram illustrating the relationship between the carriers and the sidebands in the above system.

Figure 9 is a vector diagram illustrating the addition of selected sidebands and the cancelling of the others.

In Figure 1, microwave energy from an X band frequency klystron enters a horizontal rectangular wave guide 10 and passes through a first directional coupler 11, then along the guide extension 12, through a variable attenuator 13, through a line stretcher 14, then through an amplitude modulator or polarizer 15, into the vertical end of a twisted guide section la which joins with a horizontal Wave guide, then into a shunt or central side element 17 of a magic tee, indicated generally at 18, then through a collinear arm 19 of the tee, and then out through the outlet guide section 20.

Microwave energy from the source also passes through a hybrid directional coupler 21, where it splits to go through connected guide sections 22 and 23. Sections 22 and 23 each contain a variable attenuator 13, a line stretcher 14, and a modulator 25, and section 22 continues through a twisted line element 26 and out through line or section 27 to the E or vertical arm 28 of the magic tee 18, while the energy in the twisted section 29 passes through a directional coupler 30, and into the output section 20.

The modulator 25 contains a ferrite core, and they are similar in construction to the gyrators described in the patent application of I. C. Cacheris, Serial No. 411,595, filed on February 19, 1954. The means for magnetizing the ferrite cores will be discussed hereinafter.

An energy dissipating element 31 is provided on an arm of the directional coupler 11, another dissipating arm 32 is provided on the hybrid directional coupler 21, another dissipating arm 33 is connected to a collinear arm 34 of the magic tee 18, and another dissipating arm 35 is connected to the directional coupler 30, and these dissipating elements dissipate unwanted components` of the microwave energy streams.

The incoming microwave energy stream which has been divided into three streams has each of the three energy streams brought to the same energy value by adjustment of the variable attenuators 13, so that each stream contains one third of the incoming energy from the klystron, and the three streams are adjusted by the line stretchers 14, so that we have three streams apart to be modulated by the modulator 25.

A typical modulator 25 is shown in Figure 2 to polarize the electric field in the horizontal guide section '36 so that it will pass through the vertical guidesection 37, and in Figure 3 an illustrative example of the modulator is shown, and it has a ferrite core 38 in a cylinder 39of a dielectric material, such as that generally known by the trade name of Teflon, and entered in suitable supporting ends 40, and a coil 41 of 1000 turns of Litz wire is wound about the cylinder 39 and connected to terminals 42. A The coil is covered by black linen Bakelite, and the black linen covering is given a thin coat of silver paint 43. Each ferrite core is magneticallyV biased by a direct current flowing through the coil 41, and'superimposed on this direct current circuit is an alternating voltage of a desired modulating frequency and of an amplitude variation capable of swinging the magnetic field set up by the coil from zero to a predetermined maximum value.

Figure 4 shows the modulating means for the modulators. In this figure a D.C. source 44 supplies current through isolating chokes 45 to the modulator coils 41, through three lines 46, 47 and 48, and through conventional means for adjusting the current intensity. A conventional oscillator 49 feeds through a conventional phase shifting network S0, so that we have three output voltages which are 120 apart in phase in the three lines 51, 52 and 53. These output voltages are each amplified by conventional amplifiers 54, and are fed through coupling capacitors 55 into the coils 41.

The initial bias from the D.C. source rotates the electric field about 45 and the superimposed alternating current neld on one half Wave swings the magnetic eld in the `ferrite to completely rotate the electric field 90, and the other half wave overcomes the bias and brings the magnetic field in the ferrite to zero. The relationship between the magnetic field and the swing of the electric eld is not perfectly linear, but the small departure from perfect linearity does not cause any operating trouble.

In the modulator used, the ferrite core was 1A inch in diameter and 1%/4 inches long, but these dimensions are stated for illustrative purposes only and not by way of limitation. The quality of the ferrite rod used will, of course, call for adjustments in the magnetic field used but will not change the modulation method.

The modulation may be carried out with very low alternating current frequencies, and the frequency of modulation may be stepped up to very high frequencies. The upper limit of frequency will be largely determined by the response of the coil constants to the upper frequencies. Operation at kilocycles for the alternating current modulation frequency was readily used, but there is no reason why this modulation frequency cannot be increased materially until the coil constants make it diiicult to magnetize the ferrite core` through the coil.

Referring now to the vector diagram in Figure 5, we have each microwave stream of the three streams amplitude modulated and the three carriers are represented by C. The upper sideband U revolves counterclockwise with respect to C at an angular velocity Wd while the lowersideband L revolves clockwise at a velocity of minus Wd relative to C. The carriers are assumed to be rotating counterclockwise at an absolute angular velocity W. The absolute angular velocity of U with respect to a xed coordinate system is W plus Wd, and the absolute angular velocity of L is W minus Wd. The three microwave streams are symmetrically phased and amplitude modulated 120 apart so that the carriers and upper sidebands cancel, and We have only the lower sidebands going out from the system.

The signal in the output of the system is shifted from the input signal from the source by plus or minus the modulation frequency depending on the relative phases of the input streams.

If a three-volt microwave ysignal is split into three equal signals or streamsI which are 100% amplitude modulated, the carrier in each stream will be 0.5 volt with the two sidebands of 0.25 volt each. With the three streams recombined, the total signal in the output end of the system due to the three upper (or lower) sidebands is 0.75 volt. The ratio of the output voltage to the input voltage is 0.25, or 12 db. When operating the system that has just been described at X-band and with V20 kilocycles modulation, for an input signal of 18 milliwatts the output of the transmission system was 13.5 db down. This figure compares favorably with the theoretical value of l2 db. The ferrite core used was of what is known as Ferramic A.

The output is, of course, the sum of the selected upper or lower side bands, depending upon which are used, and it will be understood that when the term single sideband is used that it means the sum of the sidebands used. The single sideband, suppressed carrier, microwave modulator just described has an inherent loss since only one sideband is used.

The dissipating arms, already referred to, on the several elements of the system, take care of reflections due to any slight unbalance of the system so that the output is only that of the single sideband desired.

A reflection system to modulate the three energy streams and to reflect back a single sideband through the input end of the system, and having the frequency of the input signal from the source plus or minus the modulating frequency, is diagrammatically shown in Figure 6. In.this system the incoming signal from a klystron X band oscillator enters the system at the incoming end of the guide 60, and enters the shunt arm 61 of a magic tee indicated generally at 62. It then leaves through a collinear arm 63, through an arm 64, through a modulator 65, and terminates in a cross-polarized shorted arm 66. The arm 64 contains a variable attenuator 67 and a line stretcher 68. It also leaves through a collinear arm 69 of the magic tee, then along an arm 70, then through a modulator 65, and terminates in a cross-polarized element 72 having an adjustable short. The arm 70 contains an attenuator 67 and a line stretcher 68. The E arm 75 of the magic tee is connected to an arm 76, which contains an adjustable transformer 77 and which terminates in a dissipating element 78. Before the incoming energy stream reaches the magic tee it is also divided by a directional coupler 79, which has an arm 80, and it passes through a Variable attenuator 81, then through a modulator 82, and terminates in the shorted crosspolarized element 83. Another arm 84 of the directional coupler 79 contains an adjustable transformer 85 and terminates in a dissipating element 86.

The modulators 65, 71 and 82 are similar to those already described, and they are controlled by the frequency modulating means also already described.

The incoming energy stream from the X type klystron is divided three ways, the three streams are adjusted to have equal energy, and they are adjusted so as to be 120 apart, so that three equal streams, 120 apart, are modulated and reflected, so that a single sideband is reflected back through the inlet end of the system, and with suppression of the carriers.

The system is frequency sensitive because of the adjustable wave guide shorting means behind the modulators. With the system tuned for optimum frequency the power output of this reflection system was about 16 db below the input.

In order to repeat signals at a fixed frequency difference from a source which is frequency modulated more than a few megacycles, the reflection system should be made broadband. Wave guide terminations which are dissipative instead of shorts would make the system less frequency sensitive, but the modulators would require more modulation power for the same carrier modulation. Waveguide terminations of a dissipative nature were not used because amplifiers with sufficient output for modulation of ythe carriers were not available.

The transmission system described and in which three equal streams of energy apart'are modulated to result in carrier suppression and to give a single sideband output may be modified to apply the same principle of operation to systems having more than three phases. It may also be modified for use with a two phase system as shown in the block diagram of Figure 7.

ln Ithe block diagram the numerals 90 are applied to a klystron oscillator, 91 and 92 are applied to directional couplers, 93 and 94 are applied to magic tees having the E arm terminated by matched terminations 95 and 96, respectively, 97 and 98 are applied to amplitude modulators, 99 are applied to a 90 phase shifter, 100 are applied to a Variable attenuator, 101 are applied to an adjustable phase shifter, and the numerals 102 are applied to the output line. y A

For the moment let us disregard the directional couplers 91 and 92 and the arm C. Power from the oscillator is equally divided in the two arms A and B by the hybrid or magic tee 93. In the arm A, this energy is amplitude modulated by the ferrite amplitude modulator 97. A vector diagram which vectorially represents the energy emerging from arm A after the amplitude modulater 97 is shown in Figure 8. In this figure A2 and A3 represent the upper and lower sidebands of the carrier A1, introduced by the modulator 97.

In arm B the phase shifter 99 introduces a 90 lag in the phase of the stream in B with reference to arm A before the signal is amplitude modulated in the ferrite amplitude modulator 98. The 90 lag in the phase of the carrier is shown in Figure 8 as B1, and B2 and B3 are the sidebands produced by the amplitude modulaltion. The audio frequency modulation which drives the amplitude modulator 98 has a 90 time phase lag, so that at the instant the sidebands of A are in phase with the carrier A1, the sidebands of B are 90 out of phase with the carrier of B, as shown in Figure 8. The modulated signals are then combined in the output arm or shunt 102 of the magic tee 94. The vector diagram for the output arm of the magic tee 93 is shown in Figure 9. It is seen that the lower sideband is not present.

Now consider the directional couplers 91 and 92 and the arm C, which couple energy at the carrier frequency into the output arm 102. The variable attenuator 100 and the adjustable phase shifter 101 are set so that the energy coupled into the final output arm is equal and opposite in phase with R1. This cancels out the carrier frequency in the final output arm. The output at 102 is the single sideband R2 which has a frequency shifted from the frequency of the oscillator 90 bythe modulating frequency of the modulators.

=In going from the modulators 97 and 98 to the magic tee 94, the energy passes through twisted line elements 103 and 104, respectively, so that the rectangular and horizontal type of wave guide may be used throughout for convenience in assembling the elements.

The means for varying the magnetic field in the ferrite cores of the modulators 97 and 98 are similar in principle to those shown in connection with the three phase system with the exception that the modulating system is now arranged to feed two-phase to the modulator coils.

The term "amplitude modulation has been used for want of a better term although the modulators do notv vonly exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claims.

We claim: l

1. A microwave reflection system having an input end, means at said input end to receive a single stream of microwave energy at a given carrier frequency and to divide said stream into a pluralitypof energy streams, means to adjust said streams for differences of phase relative to each other, means to amplitude modulate each of said phased streams at a frequency other than said given frequency by ferrite modulators, means to suppress the carriers, means to add selected sidebands of said modulated streams and to reflect outwardly through said input end a single sideband having a frequency different from said given frequency.

2. A single sideband microwave reflection system having an input end, means at said input end to receive a single stream of microwave energy at a given carrier frequency and to divide said single stream into three energy streams, variable attenuator means in the path of each stream for adjusting the energy content thereof, line stretcher means in the path of each stream for adjusting the phase relationship thereof, modulation means including ferrite modulators in the path of each stream for amplitude modulating the three streams with modulation signals having a predetermined phase relationship, reflection means in the path of each stream for reflecting back each stream after it has passed through said variable attenuator and line stretcher and modulation means, and means for recombining the reflected three streams at said input end, said variable attenuator and line stretcher means being adjusted so that the refiected streams at said input end have substantially the same energy value and are symmetrically phased apart, the carriers thereupon canceling upon recombination, and said predetermined phase relationship of said modulation signals being chosen so that the carriers at said input end are modulated 120 apart, one sideband thereupon canceling upon recombination and the other sideband being reflected outward from said input end.

References Cited in the le of this patent UNITED STATES PATENTS 1,560,505 Duncan Nov. 3, 1925 1,773,116 Potter Aug. 19, 1930 2,151,464 Curtis Mar. 21, 1939 2,500,945 Hansen Mar. 21, 1950 2,745,060 Sferrazza May 8, 1956 2,748,353 Hogan May 29, 1956 2,793,349 Crosby May 21, 1957 2,802,183 Read Aug. 6, 1957 

